In many sewing operations, especially in the clothing industry, the thread ends of a seam shall be on the underside of the fabric sewn or on the inner side of an article of clothing in order to achieve an attractive appearance of a seam. This requirement is satisfied by the use of correspondingly designed thread-cutting devices, which cut the needle thread and the hook thread under the needle plate. It must be ensured at the beginning of sewing that the loose needle thread end will be pulled onto the underside of the fabric being sewn during the first stitch formation cycle. In order for a secure connection of the needle thread and hook thread to be able to take place at the beginning of a sewing process, a reserve-side needle thread end of sufficient length must be left at the end of the seam formed previously during the thread-cutting process. However, this long needle thread end, which is necessary for reliable sewing-on, leads in turn to the formation of so-called bunches of threads of unattractive appearance on the underside of the fabric being sewn at the beginning of the seam. Since such thread bunches are unacceptable in many sewn products, especially those of high quality, these must subsequently be removed in a separate, relatively time-consuming operation.
This disadvantage is eliminated in a stitch group sewing machine known from German Patent No. DE-PS 289,806 by providing a device for shortening the needle thread end at the beginning of sewing in addition to the thread-cutting device. This additional device has a catch thread arm, which ends in a catching hook and is attached to a support wheel of the thread-cutting device, which support wheel is rotatable around a vertical axis and carries two cutting knives. A spring-loaded thread clamp, which is normally open, and by which a cam surface designed at the catch thread arm can be closed, is arranged on the underside of the needle plate. A rotary movement, which is opposite the rotary movement taking place at the beginning of a thread-cutting process, is imparted to the support wheel at the beginning of a sewing process. Due to this rotary movement, the thread catch arm introduces the needle thread end, which was pulled under the needle plate during the first stitch, into the opened thread clamp. During the further movement of the catch thread arm, the thread clamp is then closed via the cam surface, and the section of the needle thread end projecting beyond this cam surface is finally cut off, while the catching hook cooperates with a separate knife. After forming two stitches, the catch thread arm is pivoted back into the starting position, and the thread clamp is again opened.
Even though the technical expense required for embodying the device is reduced in the prior-art sewing machine due to the fact that the support wheel carrying the cutting knives of the thread-cutting device also carries the catch thread arm used to catch the needle thread end, it is now necessary to move the entire support wheel within a larger angle range than during the actual thread cutting in order to shorten the needle thread end, so that the drive mechanism also must be accordingly adapted. Therefore, the prior-art process is suitable for shortening the needle thread end mainly for stitch group sewing machines, in which the special drive movements of the support wheel can be brought about in a relatively simple manner by correspondingly designing the cam plate.